Transmission line resonators are known to comprise a transmission line that is disposed on a dielectric material. One end of the transmission line is typically short-circuited (i.e., connected to ground), while the other end is typically open-circuited and capacitively coupled to ground. The open-circuited end of the transmission line is used to interconnect the transmission line resonator with other circuitry--for example, to a transistor in an oscillator circuit.
Transmission line resonators are most commonly fabricated as either a dielectric block resonator or a planar microstrip resonator. In the dielectric block configuration, the transmission line is deposited along the inside surface of a hole in a block of alumina ceramic or barium tetratitanate, which hole extends through the center of the block. The outer five sides (i.e., the four sides parallel to the transmission line and the bottom end) of the block are typically plated with an electrically conductive material, such as copper or tin, and are connected to ground and one end of the transmission line. The top end of the block includes two areas of electrically conductive plating. The first area is located about the periphery of the top end and is connected to ground. The second area is an isolated area of plating that is connected to the open end of the transmission line. The second area of plating capacitively couples the open end of the transmission line to ground.
In the planar microstrip configuration, the transmission line is deposited, or etched, onto one side of a printed circuit board material. The other side of the printed circuit board material contains a ground plane that extends beneath the transmission line. One end of the transmission line is typically grounded via an electrically conductive throughhole and the other end is capacitively coupled to ground via one, or more, discrete capacitors.
In either of the above resonator configurations, the resonant frequency of the transmission line resonator is determined by calculating the parallel impedance produced by the coupling capacitance and the effective impedance of the short-circuited transmission line (typically an inductive reactance), and setting the parallel impedance equal to infinity. The resonant frequency is given by the following equation: EQU f.sub.r =1/(2.pi.)(L.sub.t C.sub.c)1/2 (Equation 1)
where f.sub.r is the resonant frequency, L.sub.t is the effective inductance of the short-circuited transmission line, and C.sub.c is the coupling capacitance.
Adjustment of the resonant frequency is typically accomplished using one of a variety of tuning techniques depending on the type of transmission line resonator. For example, a dielectric block resonator might be tuned to a particular resonant frequency by removing the plating that is attached to the open end of the transmission line. This plating removal is typically performed using a laser or a sand blaster. When a laser is used, the laser vaporizes a path in the plating that effectively cuts away a portion of the plating that is connected to the open end of the transmission line, thereby reducing the coupling capacitance and increasing the resonant frequency. Thus, laser tuning permanently adjusts the resonant frequency in only one direction (i.e., increasing). In addition, laser tuning typically reduces the quality factor (Q) of the resonator, especially when the dielectric block is constructed of a material with a high dielectric constant (e.g., greater than 30). That is, heating a localized area of the dielectric material increases the dielectric loss tangent of the dielectric material and lowers the effective Q of the dielectric material. Further, laser tuning requires specialized equipment (i.e., a laser) to complete the process.
Similar to laser tuning, sand blasting removes a portion of the plating that is connected to the open end of the transmission line. This is accomplished by propelling pressurized sand at the portion of the area that is to be removed. Unlike laser tuning, sand blasting does not degrade the resonator's Q; however, sand blasting is a dirty process that requires specialized propulsion and vacuum equipment.
A third known resonant frequency adjustment technique commonly used to tune dielectric block resonators is drilling. With this technique, recesses or holes are drilled in the dielectric block adjacent to the transmission line to alter the effective dielectric constant of the dielectric block, thereby changing the effective impedance of the transmission line. Similar to laser tuning and sand blasting, the drilling process permanently adjusts the resonant frequency in only one direction and requires specialized equipment.
To adjust the resonant frequency of a typical planar microstrip transmission line resonator, the value of the discrete coupling capacitor, or the number of discrete coupling capacitors, is increased or decreased depending on whether the resonant frequency is to be decreased or increased, respectively. Similar to the aforementioned techniques, this method requires specialized equipment (e.g., a soldering iron) and results in an essentially permanent alteration of the resonant frequency. That is, in order to reverse the direction of the resonant frequency adjustment, the specialized equipment must be employed again.
Therefore, a need exists for a method and apparatus to adjust the resonant frequency of a transmission line resonator assembly while facilitating dynamic, bi-directional frequency adjustment. Further, such a method that resulted in effective tuning while maintaining resonator Q, and without requiting the use of specialized equipment, would be an improvement over the prior art.